This invention relates to a method and apparatus for enriching the iron carbonyl content of a recycle gas stream, and more particularly to a method and apparatus for receiving a recycle gas from an iron carbonyl decomposition or reaction process and enriching the iron carbonyl content with the gas to enable reuse of the gas stream in the iron carbonyl decomposition or reaction process.
It has previously been suggested that highly pure metallic iron may be produced under the proper conditions by passing carbon monoxide over reduced iron containing material to form iron carbonyl, and then decomposing the iron carbonyl to deposit iron and release the carbon monoxide. More recently, it has been suggested that iron carbonyl decomposition or reaction processes may be useful in the desulfurization of hydrocarbons such as disclosed in U.S. Pat. No. 2,756,182, in the removal of sulfur during the gasification of coal, such as disclosed in U.S. Pat. No. 2,691,573, in the desulfurization of petroleum crude and primary refinery products, such as disclosed in U.S. Pat. No. 3,996,130, in the removal of pyrite and ash from coal, such as disclosed in U.S. Pat. No. 3,938,966, and in the removal of organic sulfur from coal, such as disclosed in U.S. Pat. No. 4,146,367.
A typical process for producing a metallic carbonyl entails passing carbon monoxide, or gases containing a substantial portion of carbon monoxide, over the metal of which the carbonyl is to be formed. The metal to be acted upon by carbon monoxide is typically obtained by gaseous reduction of an oxide of the metal. Although other carbonyls may be formed in this manner, substantial commercial production appears to have been limited to nickel carbonyl, since the reaction may take place at relatively low pressure and temperatures, as for example at atmospheric pressure and a temperature of about 40.degree. C. to about 50.degree. C. Iron carbonyl, on the other hand, is much more difficult to form, and is typically produced by the reaction of carbon monoxide with iron generally obtained by the reduction of iron ore, but at temperatures and pressures much higher than are required for the production of nickel carbonyl from reduced nickel. For example, temperatures on the order of 175.degree. C. or higher and pressures in the range of from 100 to 200 atmospheres, or even as high as 2,000 atmospheres, have been employed in attempts to make the reaction proceed sufficiently rapidly to make iron carbonyl production efficient.
Several attempts have been made to efficiently produce iron carbonyl by modifying reaction conditions. For example, U.S. Pat. No. 1,614,625 discloses a process for producing iron carbonyl by passing carbon monoxide under a pressure of about 200 atmospheres over iron metal at a temperature of about 200.degree. C. U.S. Pat. No. 1,759,268 discloses a process for producing iron carbonyl by passing carbon monoxide at a pressure of about 50 atmospheres or more over reduced oxides of iron at a temperature of about 100 to about 200.degree. C. at a sufficient velocity to prevent deposition of iron carbonyl on the iron oxide material. U.S. Pat. No. 1,783,744 discloses a similar process wherein higher gas velocities are utilized in conjunction with lower reaction temperature and pressure conditions. U.S. Pat. No. 1,828,376 also utilizes high gas velocities to produce iron carbonyl by passing carbon monoxide at a pressure of between 50 and 120 atmospheres over porous iron lumps at a temperature between 90.degree. and 100.degree. C.
Other attempts at the efficient production of iron carbonyl have utilized modified reaction materials. For example, U.S. Pat. No. 2,086,881 discloses a process for producing iron and nickel carbonyl from sulfur bearing matte materials preferably between 140.degree. and 300.degree. C. and at pressures preferably of 50, 100 or 200 atmospheres or even more. U.S. Pat. No. 3,112,179 discloses a process for preparing iron and nickel carbonyl by mixing nickel oxide powder with sponge iron to form a mixture containing 50 to 98% by weight of sponge iron and 50 to 2% by weight of nickel oxide powder, pelletizing the mixture, reducing the oxides in the pellets, and then passing a stream of carbon monoxide through the pellets at a temperature of 70.degree. to 170.degree. C. while maintaining the carbon monoxide pressure at a sufficiently high level to prevent substantial decomposition of nickel carbonyl.
Still other attempts at efficiently producing iron carbonyl have suggested that the presence of sulfur in an active form may increase the efficiency of iron carbonyl production. However, the form of the sulfur bearing material utilized in such a process appears to be critical in determining efficiency of iron carbonyl production. For example, as disclosed in U.S. Pat. No. 2,378,053, ". . . although as has been recognized, sulfur in the form of sulfides such as nickel sulfide is effective to increase the velocity of the reaction involved in the production of nickel carbonyl from reduced nickel, nevertheless the addition of solid sulfides to reduced iron such as described hereinbefore is not effective. Furthermore, gaseous sulfides have been found to be ineffective also in view of the fact that there is an excessive local action near the inlet port for the gaseous sulfides and relatively ineffective action at points remote from the gas inlet. Thus, it is manifest that sulfides are not effective in increasing the velocity of the reaction between iron and carbon monoxide to produce iron carbonyl". U.S. Pat. No. 2,378,058 then discloses that the reaction between reduced iron and carbon monoxide can be accelerated by treating the iron containing material with a soluble solution of heavy metal sulfates prior to reduction of the iron containing material.
While the basic reaction of carbon monoxide gas with reduced iron containing material to form iron carbonyl has been known for many years, the prior art processes for production of iron carbonyl have not proceeded with sufficient efficiency to enable substantial commercial production, or have entailed economically prohibitive reaction conditions or material treatment prior to production. Thus, several proposed industrial processes involving the decomposition or reaction of iron carbonyl, as heretofore described, have not been commercially feasible.
It has now been determined that iron carbonyl may be produced in a highly efficient manner enabling its use on a commercial scale in an iron carbonyl decomposition or reaction process by cooling a recycle gas stream produced in an iron carbonyl decomposition or reaction process to a temperature of about 5.degree. to about 15.degree. C., adding carbon monoxide to the cooled gas stream to produce a carbon monoxide enriched gas stream, compressing the carbon monoxide enriched gas stream to a pressure of about 20 to about 38 atmospheres under conditions suitable to prevent the decomposition of substantial amounts of iron carbonyl in the carbon monoxide enriched gas stream, and contacting the compressed gas stream at a temperature of about 65.degree. to about 160.degree. C. with a reduced iron containing material in the presence of an iron carbonyl production enhancing amount of hydrogen sulfide under conditions suitable to producing substantially condensed iron carbonyl. Preferably, the recycle gas stream is split into first and second portions, the first portion is cooled, enriched with carbon monoxide, compressed and contacted with the reduced iron containing material to produce substantially condensed iron carbonyl, and at least a portion of the condensed iron carbonyl is vaporized and combined with the second portion of the recycle gas stream to produce an iron carbonyl enriched recycle gas stream. The iron carbonyl enriched recycle gas stream may then be reintroduced into the iron carbonyl decomposition or reaction process.